1. Field of the Invention
The present invention relates to a head drum apparatus for writing and reading signals on magnetic tape and a magnetic recording-reproducing apparatus having the head drum apparatus. More specifically, the present invention relates to a helical scanning magnetic reproducing head using an MR head, a head drum apparatus, and a magnetic recording-reproducing apparatus having the head drum apparatus.
2. Description of Related Art
In recent years, as the amount of information to be handled increases, there is an increasing need for further improving recording densities for a magnetic recording-reproducing apparatus that records and reproduces data on magnetic tape. It is absolutely necessary to use an MR (Magneto Resistive) head instead of a conventional inductive head as a magnetic head for reading signals. The MR head is a magnetic head that reads a signal recorded on a magnetic recording medium using the magnetoresistive effect of an MR element. The MR head can provide high sensitivity of signal detection and large reproduction output. Accordingly, the MR head can easily reduce a recording track width on the magnetic tape, increase the recording density in a linear direction, and provide high-density recording and reproduction.
Generally, the MR head is characterized by susceptibility to electrostatic discharge and heat attack compared to inductive heads. FIG. 4 shows results of measuring ESD (Electrostatic Discharge) breakdown voltages at MR heads. FIG. 4A shows the measurement result of an AMR (Anisotropic Magneto Resistive) head. FIG. 4B shows the measurement result of a GMR (Giant Magneto Resistive) head.
With respect to measured values in FIG. 4, an HBM (Human Body Model) is used to measure ESD breakdown voltages. FIG. 4 shows the relationship between a voltage applied to the device and a resistance when a 100 pF capacitor is charged and then discharged with the resistance of 1.5 kΩ. According to the measurements, the AMR is supplied with an ESD breakdown voltage of approximately 230 to 240 V. The GMR is supplied with an ESD breakdown voltage of approximately 30 to 40 V.
Under normal conditions, friction, contact, induction, or the like easily generates a charged voltage of several kilovolts or more on an insulator such as plastic, nylon, vinyl, etc. For example, a high-resistance synthetic resin material is often used to form a conventional cassette case for taking up and storing magnetic tape. Such cassette case is easily electrostatically charged while a user handles it, for example, due to friction with a packaging material made of artificial fiber, friction with parts when the cassette case is loaded into the magnetic recording-reproducing apparatus, etc.
An ABS resin is one of synthetic resin materials used for cassette cases. For example, an ABS resin with the surface resistance of approximately 1016Ω/sq generates a charged voltage of 1.5 to 2 kV or more. It takes three minutes or more to halve the charged voltage. This charged voltage value far exceeds the MR head's withstand voltage. In addition, since the time to halve the charged voltage is long, the static electrification, once charged, hardly attenuates. If the magnetic tape in the electrostatically charged cassette case touches the MR head, a large amount of current flows through the MR head, possibly causing an electrostatic discharge damage.
A conventional MR head uses a head substrate comprising an MR element sandwiched between magnetic shielding films or insulating films. The protective substrate uses a conductive material such as Al2O3—TiC with the electrical resistivity of approximately 2×10 −3Ωcm. The magnetic head is electrically connected to the drum apparatus so that the head substrate and the protective substrate become the ground potential. When the electrostatically charged magnetic tape touches the MR head, the electric charge does not flow through the MR element, but through the head substrate and the protective substrate for discharge.
When the head substrate and the protective substrate are made conductive as mentioned above, however, a high voltage is applied to the MR element due to approach, contact, etc. of a charged substance from the outside. When the electrostatic change is discharged between these substrates, a very large discharge current is generated because the substrates have a low electric resistance. Accordingly, the discharge current flows through the MR element to cause an electrostatic discharge damage.
FIG. 5 schematically shows a discharge on the conventional MR head.
FIG. 5 shows a structure example of an MR head 50 against a magnetic tape's contact surface. The MR head 50 is structured to arrange an MR element 50c, and a pair of shielding films 50d and 50e made of a soft magnetic material between a head substrate 50a and a protective substrate 50b that are both conductive. Insulating films 50f and 50g are formed between a head substrate 50a and a protective substrate 50b and between shielding films 50d and 50e. The MR element 50c is formed between the shielding films 50d and 50e via insulating films 50h and 50i to constitute a reproducing magnetic head section. The MR element 50c is connected to a power supply terminal (not shown) through a conducting wire etc. The MR element 50c is powered from the power supply terminal to read data recorded on the magnetic tape.
The MR head 50 is fixed on a base metal (not shown). The base metal is fixed on a rotating drum (not shown) to mount the MR head 50 thereon. The base metal is made of conductive metal. The head substrate 50a and the protective substrate 50b are electrically connected to the base metal through conductive paste for example. The base metal is fixed on the rotating drum with a metal fixing screw or the like, for example. The rotating drum is connected to the ground in the apparatus to allow the head substrate 50a and the protective substrate 50b to be equal to the ground potential. Accordingly, when the electrostatically charged magnetic tape touches or approaches the head, for example, the magnetic tape is discharged through the head substrate 50a and the protective substrate 50b. 
Concerning the MR head 50, a charged substance such as rubbed artificial fiber may touch or approach the power terminal during a production line process, for example. In such case, the electric charge moves to the power supply terminal from the charged substance, increasing the voltage of the MR element 50c. At this time, the electric field concentrates on the shielding film 50d, and then on the head substrate 50a from the MR element 50c touching the magnetic tape's contact surface, thus increasing an electric field strength. Since the insulating films 50f through 50i are thin on the magnetic tape's contact surface, a dielectric breakdown occurs on these films. A discharge current flows into the head substrate 50a from the MR element 50c via the shielding film 50d. 
Since the head substrate 50a has a low electric resistance at this time, an excessive current flows into the head substrate 50a from the MR element 50c. Consequently, discharged traces 51a, 51b, 51c, and 51d are formed at discharged locations on the MR element 50c, the shielding film 50d, and the head substrate 50a. 
Further, when the charged substance touches or approaches the magnetic tape contact surface of the MR head 50, an electric discharge is applied to the head substrate 50a or the protective substrate 50b to cause an electrostatic discharge damage. FIG. 6 is a graph showing an electric current waveform when the conventional MR head 50 is subject to the experiment on discharging.
FIG. 6 shows currents generated when a voltage-applied probe is moved close to the magnetic tape contact surface of the MR head 50 for a discharge. The applied direct-current voltage is up to 3 kV. As seen from the graph, it is possible that an absolute value for the current exceeds 1 A during a discharge. The MR element 50c is easily subject to meltdown and the like. In addition, a discharge current destroys the shielding films 50d and 50e, the head substrate 50a, and the protective substrate 50b. 